This invention relates to a torque sensor for producing signals that represent the torque applied to stationary and/or slowly rotating shafts. In particular, this invention is designed to produce a signal representing the axle torque of an off-highway vehicle, such as a farm tractor or crawler tractor.
With the increasing cost of fuel, tractor owners today are interested in obtaining a maximum productivity from their equipment at minimum operating cost. One significant performance indicator of a tractor is its tractive efficiency. Tractive efficiency is defined as the measure of power utilization as the percent of wheel output power being used to pull the implement. Computing its value requires a continuous measurement of axle torque in addition to other signals representing velocities and forces associated with the tractor and implement. This invention describes a torque sensor which could be used for continuously producing a signal that is proportional to the tractor's axle torque.
There are a number of techniques that have been developed using strain gauges and shaft twist measurements for sensing the torque of stationary and rotating shafts. However, most of these sensors are not usable for situations where the shaft speeds are relatively low (i.e. below 100 rpm) and where the environment is severe in vibration, temperature range, and contamination.
The more common type of torque sensors uses strain gauges. These designs have the disadvantage of requiring slip-rings, such as shown in U.S. Pat. No. 4,096,743, to power the sensor elements and to transmit the output signal.
This invention relates in particular to a torque measuring system that employs a thin wall cylinder disposed about the shaft and is driven by electro-magnetic coils for oscillation at its natural frequency. Such a torque measuring system is disclosed in U.S. Pat. No. 4,099,411 which issued to George A. Woolvet. In the Woolvet patent, the frequency of vibration of the thin-wall cylinder varies according to the loading on the cylinder. The frequency of vibration of the cylinder wall depends on the longitudinal strain applied to the cylinder by the loading means.
The longitudinal loading utilized in the Woolvet invention has the benefit of providing higher sensitivity but the disadvantage of responding to shaft bending. The Woolvet invention could not be utilized for measuring the axle torque of any vehicle having high bending loads, such as a farm tractor endures. This invention employs an oscillating thin-wall cylinder arrangement for measuring torque but avoids the shortcomings of the Woolvet invention by employing compensating means for minimizing the temperature and shaft bending effects.